When I got my N900, I got a range of cheap accessories off ebay. Of them, a couple ended up being used.
1. Screen protector. I got a 3/$0.99 pack, and they actually fit nicely. Getting one on without having dust or hair between it and the N900's screen was a challenge though.
I ended up pulling my third one off because I -still- got some dust between it. Also, screen protectors are impossible to clean with "normal" methods, like a paper towel - However, a little soap and water works -wonders-. Leave it slightly damp, put it on your N900, then wipe all the excess water off before it can get under the bezel.
Also, these do tend to scratch up; I'm gonna need to get a new set soon.

2. Clear plastic hard-case. I got two of these at different times; One fit too loosely, and I ended up breaking one of the plastic tabs holding it on. The second was a bit tighter(same mfgr, probably a different batch), and I ended up using it for quite a while. However, with one of these cases on, it feels significantly larger in your hand, and won't fit most N900 holsters/cases. It has an added benefit of preventing accidental button presses.

3. N900 "cross" bag/case. It's a ~$5 fabric case with two pockets, and a "cross" of fabric sewn onto the front-top. It's got a belt-loop, and fits the N900 - even with a hard case - nicely. The front pouch has just enough space for a small USB cord, extra battery, and a set of earbuds, and I can fit the whole thing in my pocket.

4. Mobile charger. My dad got one of these iGO units from Radio Shack for a previous phone; I added a ~$5 USB adapter tip, and I've been using it to charge all my stuff. For my N900, I had to take the USB charger tip/adapter apart and solder the middle pins together; it worked perfectly after that.

5. microSD card. More space is always good, and you can remove it. Good for keeping backups and important stuff on, just in case you make your N900 need a reflash.